An MS/MS analysis (also called the “tandem analysis”), which is one of the mass spectrometric techniques, has been widely used in recent years, mainly for the purpose of identifying substances having high molecular weights and analyzing their structures. A triple quadrupole mass spectrometer (also called the “tandem quadrupole mass spectrometer” or otherwise) is one type of mass spectrometer capable of MS/MS analyses and is popularly used since it has a comparatively simple structure and is inexpensive.
A triple quadrupole mass spectrometer normally has a collision cell for dissociating an ion by collision-induced dissociation, which is placed between the two quadrupole mass filters provided on the front and rear sides of the cell, respectively. The front quadrupole mass filter selects a precursor ion having a specific mass-to-charge ratio from among various ions derived from a target compound, while the rear quadrupole mass filter separates various product ions produced from the precursor ion according to their mass-to-charge ratios. The collision cell is a box-like structure which is hermetically sealed to a comparatively high degree, into which an inert gas (such as argon or nitrogen) is introduced as the collision gas. The precursor ion selected by the front quadrupole mass filter is given an appropriate amount of collision energy and introduced into the collision cell. Within this collision cell, the ion collides with the collision gas and undergoes the collision-induced dissociation process, whereby the product ions are produced.
The dissociation efficiency of the ion within the collision cell depends on the amount of collision energy possessed by the ion introduced into the collision cell, the pressure of the collision gas in the collision cell (hereinafter, the “collision-gas pressure” should mean “the pressure of the collision gas in the collision cell” unless otherwise specified), and other factors. Therefore, the detection sensitivity of the product ion which has passed through the rear quadrupole mass filter also depends on the amount of collision energy and the collision-gas pressure.
The measurement using a triple quadrupole mass spectrometer is often performed in a multiple reaction monitoring (MRM) mode in which the mass-to-charge ratio at which the ions are allowed to pass through is fixed in each of the front and rear quadrupole mass filters in order to determine, with a high level of accuracy and sensitivity, the quantity of a known compound. Therefore, the collision-gas pressure in a triple quadrupole mass filter is normally designed to be set at a value (usually, a few mTorr) previously adjusted by the manufacturer so that the highest possible level of detection sensitivity will be obtained in the MRM measurement mode. However, the collision-gas pressure which gives the high level of detection sensitivity varies depending on the kind of compound. Therefore, under the condition that the collision-gas pressure is always adjusted at one value in the previously described manner, although the high level of detection sensitivity is obtained for some compounds, the level of detection sensitivity for other compounds will inevitably be low.
To overcome this problem, some triple quadrupole mass spectrometers have the function of allowing analysis operators (users) to freely adjust the collision-gas pressure (see Patent Literature 1). In this type of apparatus, to realize a high level of detection sensitivity for a specific compound, the analysis operators themselves need to investigate the optimum collision gas for that compound. A typical procedure for determining the optimum collision-gas pressure in a conventional triple quadrupole mass spectrometer is as follows:
Initially, the analysis operator prepares a plurality of method files for different levels of collision-gas pressure (a method file is a program file which defines the analysis conditions including the collision-gas pressure, the voltage applied to each component of the apparatus and other parameters). Subsequently, the operator repeatedly performs a preliminary measurement for a sample containing the target compound, using each of the method files, to collect signal intensity data for an ion derived from the target compound, i.e. a set of data which show a change in the signal intensity for a change in the collision-gas pressure. Based on the measurement result, the operator locates the collision-gas pressure which gives the highest signal intensity, and determines that this gas pressure is the optimum collision-gas pressure for that compound.